1. Field of the Invention
This invention relates to improvements in a throttle valve device for variably controlling an amount of intake air to be supplied to an engine of an automotive vehicle or the like in accordance with an amount of operation of an accelerator, and more particularly to the improvements in the throttle valve device of the type wherein a throttle valve is rotated to open and close by using an actuator such as an electric motor.
2. Description of the Prior Art
Hitherto a variety of throttle valve devices for an internal combustion engine have been proposed and put into practical use. An example of such throttle valve devices is arranged to be disposed in an intake air passageway leading to cylinders of the engine and include a throttle body in which a part of the intake air passageway is formed. A throttle valve is rotatably disposed through a valve shaft in the throttle body and adapted to open and close the part of the intake air passageway in accordance with a rotational movement of the valve shaft. An electric motor is provided in the throttle body to drive the valve shaft. Additionally, a reduction gear mechanism is provided between the electric motor and the valve shaft to transmit a driving force of the electric motor to the valve shaft upon making a rotational speed reduction. Such a throttle valve device is disclosed in Japanese Patent Publication (Kohyo) No. 2-500677 and Japanese Patent Publication No. 4-203219.
However, drawbacks have been encountered in the above-discussed conventional throttle valve device, as set forth below. The conventional throttle valve device is provided with first biasing means for always biasing the throttle valve in a direction toward a fully closed position, and second biasing means for biasing the throttle valve in the opposite direction to that by the first biasing means so as to bias the throttle valve in a direction toward an intermediately or partly opened position between the fully closed position and a fully opened position.
In order to open the throttle valve by the electric motor, the rotational force of the electric motor is transmitted through the reduction gear mechanism to the valve shaft of the throttle valve so as to rotate the valve shaft against the bias of the first biasing means. The throttle valve changes the amount of intake air to be supplied to the engine, in accordance with the opening degree of the throttle valve, thereby variably altering a rotational power output of the engine. Additionally, for example, in case that the engine is stopped (supplying no power to the electric motor) or that the electric motor is in trouble, when the throttle valve is rotated toward the fully closed position over the intermediately opened position under the action of the first biasing means, the rotation force in the opposite direction to that by the first biasing means is applied to throttle valve by the second biasing means, thus keeping the throttle valve at the intermediately opened position against the bias of the first biasing means.
Thus, the above conventional throttle valve device is configured such that the throttle valve is always biased toward the closing position by the first biasing means while is biased toward the intermediately opened position by the second biasing means. These first and second biasing means are constituted respectively of two springs, and therefore the number of parts is increased thereby lowering operational efficiency during assembly of the throttle valve device. Additionally, since the two springs are provided within the throttle body, restriction in layout arises thereby making it possible to small-size and compact the whole throttle valve device. Furthermore, a load torque (the biasing force of the first and second biasing means) to be applied to the electric motor is changed between a case where throttle valve is driven in an opening direction over the intermediately opened position and a case where the throttle valve is driven in a closing direction over the intermediately opened position. Therefore, an opening degree control or adjustment of the throttle valve under the action of the electric motor unavoidably becomes ununiform.